Tonga was plunged into mourning Monday at the death of King George Tupou V, an eccentric reformer who relinquished absolute power to bring democracy to the impoverished South Pacific nation.

The death aged 63 of the monocle-wearing king, who had a penchant for flamboyant military uniforms and driving around in a London taxi, had engulfed the country in a "black stormcloud", Prime Minister Lord Tu'ivakano said.

In a national address on public radio, Tu'ivakano called on Tongans to pray for the royal family during a sad time for the nation, according to a translation of his remarks by Radio New Zealand.

Steve Burling, an expatriate resort operator on the main island of Tongatapu, said many people, particularly older Tongans, were deeply upset at their monarch's passing.

"There's a lot of in town people wearing black, it's all everyone's talking about," he told AFP.

The cause of the king's death at a Hong Kong hospital on Sunday was not immediately known but he underwent surgery to have a kidney removed in Los Angeles last year after a cancerous tumour was discovered.

Before ascending to the throne, the king was best known for his globe-trotting lifestyle and elaborate uniforms, including colonial-era pith helmets and jackets with elaborate gold braiding.

Often sporting a monocle, he was driven around the capital Nuku'alofa in a black London taxi, with his hobbies including sailing model boats in his swimming pool and staging mock wars with battalions of toy soldiers.

His eccentricities sometimes led to criticism he was out of touch with Tonga's 115,000 population, more than a quarter of whom live below the poverty line, but he also won praise for his democratic reforms.

"He believed that the monarchy was an instrument of change and can truly be seen as the architect of evolving democracy in Tonga," New Zealand Prime Minister John Key said. "This will be his enduring legacy."

Australian Prime Minister Julia Gillard said her country had lost "a great friend" who guided his country through a critical period of constitutional change.

"Tonga's first truly democratic elections, held in November 2010, set the country on a new course," she said.

Tupou V promised reforms shortly before he was sworn in as king of one of the world's last absolute monarchies in September 2006 following the death of his father King Taufa'ahau Tupou IV.

The need for reform took on added urgency when riots two months later left eight people dead and much of downtown Nuku'alofa destroyed, delaying his coronation until 2008 as details of the constitutional revamp were finalised.

Within two years, the Oxford University-educated monarch had made good on his pledge for democracy and the people of Tonga voted in their first popularly elected parliament in 2010, ending 165 years of feudal rule.

Despite the political changes, the country spread over more than 170 islands remains mired in poverty, its debt-laden economy reliant on foreign aid, remittances sent home by expatriates, and a faltering tourism industry.

China helped bankroll rebuilding efforts in Nuku'alofa after the riots, with Australia-based thinktank The Lowy Institute estimating last year that loans from Beijing accounted for 32 percent of the country's gross domestic product.

Tupou V will be succeeded by his brother Crown Prince Tupouto'a Lavaka, who was at the king's bedside when he died in Hong Kong.

The 52-year-old heir to the throne, a church-going military man who is married with three children, is seen as far more conservative than his larger-than-life brother

Currently Tonga's ambassador to Australia in Canberra, he served as prime minister from 2000 until 2006.

His coronation and the late king's funeral are likely to be lavish affairs that could place further strain on the country's finances.

When Tupou V was crowned, more than 200 nobles and chiefs presented dozens of slaughtered pigs and hundreds of baskets of food in tribute, with the new king offered a bowl of kava, a mild narcotic drink, to signify his sovereignty.

Tu'ivakano said funeral arrangements for Tupou V were yet to be finalised.

Tonga's eccentric king achieved serious reform
Nuku'Alofa (AFP) March 19, 2012 - Tonga's late King George Tupou V was renowned for his love of elaborate military uniforms and a jet-setting lifestyle but he also achieved major political reform, experts said Monday.

The South Pacific monarch died aged 63 in a Hong Kong hospital on Sunday and will be succeeded by his brother Crown Prince Tupouto'a Lavaka, 52, seen as far more conservative than his flamboyant sibling.

Tupou was viewed as something of a dilettante when he was sworn in as absolute monarch in September 2006, more interested in the pomp and ceremony of military parades and over-the-top uniforms featuring acres of gold braid.

His penchant for being driven around the poverty-stricken streets of Nuku'alofa in a black London cab, sailing model boats in his swimming pool and waging mock wars with toy soldiers added to an oddball image.

But the Oxford-educated monarch saw the need to reform his country's antiquated constitution even before taking power and moved decisively to end 165 years of feudal rule after riots rocked the capital in November 2006.

The civil unrest, sparked by a mixture of frustration at the slow pace of reform and opportunistic looting, left eight people dead and much of the downtown area of Nuku'alofa in smouldering ruins.

Delaying his coronation until 2008, Tupou entered negotiations with reformers on changes that would bring majority government to Tonga for the first time.

Malakai Koloamatangi, an expert in Tongan politics at New Zealand's Canterbury University, said Tupou's achievements should not be overshadowed by his eccentricities.

"He wasn't afraid to compromise and he fulfilled his promise to grant more freedoms in just a few years on the throne," he told AFP.

Times of Tonga editor Kalafi Maloa said attitudes to the monarch had shifted during his reign, bringing him closer to the people.

"Over the last several years he has grown in terms of his popularity," he said. "He has over this short time... really won the hearts of the Tongan people."

Tupou V was born on May 4, 1948, the eldest of four children, and educated in Switzerland, New Zealand and Britain, where he attended Oxford University and the British army's elite military academy at Sandhurst.

He was the first Tongan to ever earn a university degree, according to the official Tongan palace website, and acted as the country's foreign minister from 1979 to 1988.

As monarch, he maintained a globe-trotting lifestyle, sometimes prompting criticism that he did not spend enough time in his homeland.

One of his last public appearances was an audience with Pope Benedict XVI in the Vatican last month.

The rugby-mad monarch died in Hong Kong as the southern Chinese city prepared to host its annual rugby sevens competition this coming weekend.

His immediate cause of death was not known but he underwent surgery in Los Angeles last year to remove a kidney after a cancerous growth was discovered.

Koloamatangi said the late king's successor, former Crown Prince Tupouto'a Lavaka, was far more strait-laced than Tupou and was unlikely to repeat his extravagances.

"He's a serious military man, he's married and he's a church-goer, a lay preacher I believe," he said.

"Things are likely to become a lot more orderly under him."

The new monarch comes to the throne after spending the past three-and-a-half years as his nation's first ambassador to Australia in Canberra, where he is understood to have strong connections.

He served as prime minister from 2000 until his sudden resignation in 2006, when he was replaced by Feleti Sevele, the first commoner to be appointed to the job.

Koloamatangi said respect for the monarch remained deeply entrenched on Tonga's hierarchical society and the new king would retain a major role in the country of 115,000.

But he said the changes initiated by Tupou had forever changed Tongans' relationship with the once all-powerful monarchy.

"Those reforms are irreversible," he said. "Even if the monarch wanted to go back he couldn't because he no longer has the power.

"There's also a strong will now for these political reforms to continue.".